Improve documentation on sending delayed emails

There are a couple of gotchas in the existing documentation
about the `send_devise_notification` hook.

1.  The `after_commit` callback can be called multiple times
    so you should clear the array otherwise any additional
    invocations will trigger extra copies of the email.

2.  The `after_commit` callback is only called when a record
    is created or updated so you need to check for `new_record?`
    or `changed?` before adding it to `pending_notifications`
    otherwise it's okay to send it immediately.

The `new_record? || changed?` condition is necessary because
the latter isn't always true for new records, e.g:

    >> User.new.changed?
    => false

[ci skip]
Merge pull request #2311 from pixeltrix/improve-delayed-email-documentation

Improve documentation on sending delayed emails